Teclast X98 Air 3G: unbricking a Bay Trail tablet

While I was trying to boot a vanilla linux kernel on the Teclast X98 Air 3G I messed up a bit with the UEFI firmware settings: the battery was not recognized anymore, the tablet was rebooting right after showing the UEFI firmware boot screen, and USB-host was not working either; so no keyboard for accessing the firmware menu to restore the default settings, and no ADB for running flashrom or the Intel FPT to reflash the firmware with.

Teclast X98 Air 3G, internals
Teclast X98 Air 3G, mainboard
ISP with a logic level converter, top
ISP with a logic level converter, angle